overview of atlas results - indico · heavy-ion data sets • run 1 (2010-2013) provided collisions...

Iwona Grabowska-Bold (AGH UST, Cracow) on behalf of the ATLAS Collaboration Wuhan, China, Sept 23rd, 2016

Overview of ATLAS results

Introduction

•  Use variety of final states to provide insight into various stages of heavy-ion (HI) collision •  Hard probes:

•  Colorless objects e.g. electroweak bosons – standard candles in the medium •  Color objects e.g. jets, hadrons – insight into partonic energy loss in the QGP

•  Bulk particle production: •  Sensitivity to initial geometry, initial conditions, collective behavior, etc

•  Disentangle initial- and final-state effects using p+Pb and pp systems •  Understand the origin of ridge in the small systems

•  Perhaps find a connection between p+Pb and Pb+Pb systems

Overview of ATLAS results, Sept 23rd, 2016 2

One of the main goals of heavy-ion physics is to study the QGP

ATLAS detector 3

Three main components: inner tracker, electromagnetic (EM) and hadronic (HAD) calorimeters, and muon system

Full azimuthal acceptance

Sub-detectors |η| coverage

Inner Tracker <2.5

Muon Spectrometer

EM Calorimeter <3.2

HAD Calorimeter <4.9

FCal ETàcentralityàNpartNcoll

Centrality

In Pb+Pb: total FCal ET In p+Pb: FCal ET on Pb-going side

Overview of ATLAS results, Sept 23rd, 2016

Heavy-ion data sets •  Run 1 (2010-2013) provided collisions of pp, p+Pb and Pb+Pb systems

•  In addition to the bulk of pp data at 0.9, 7 and 8TeV at high luminosity •  In Run 2 (2015-present) the center-of-mass energy has almost been

doubled •  Focus of this presentation on Run-2 measurements

System Year sqrt(sNN) [TeV]

Pb+Pb 2010 2.76 9 µb-1 Pb+Pb 2011 2.76 0.14 nb-1 p+Pb 2012 5.02 1 µb-1 p+Pb 2013 5.02 29 nb-1

pp 2013 2.76 4 pb-1 pp 2015 13 75 nb-1 pp 2015 5.02 28 pb-1

Pb+Pb 2015 5.02 0.49 nb-1

Performance in Pb+Pb collisions in Run 2 •  Dedicated trigger strategies developed for HI collisions

•  Many effort put to preserve constant trigger efficiency w/ centrality

•  Also offline particle identification reoptimized for HI collisions

See Radim Slovak’s poster (trigger) See Jakub Kremer’s poster (offline)

Hard probes in p+Pb and Pb+Pb

systems

Z boson in pp and p+Pb systems •  Z boson production with decay via muon channels studied in pp 5TeV

data •  Cross section in the fiducial volume of 66<Mµµ<116 GeV, |yZ|<2.5 is:

590±9 (stat.) ±11 (syst.) ±32 (lumi.) pb •  In good agreement with the NNLO calculation using the CT14 PDF:

573.77+13.94-15.96 pb

•  RpPb extracted using the old p+Pb result and new pp reference data •  Enhancement in the Pb-going direction (negative yZ) and depletion in the p-going

direction (positive yZ) – observation consistent with nPDF predictions

ATLAS-CONF-2016-107 Phys. Rev. C 92, 044915 (2015)

See Mirta Dumancic’s talk

RpPb of high-pT hadrons •  Hint of enhancement of particle production at high-

pT in Run-1 data where pp yields were interpolated from 2.76 and 7TeV

•  Now with new pp 5TeV data, spectra of charged particles with pT up to 100GeV measured •  Nuclear modification factor RpPb for old p+Pb data

(ATLAS-CONF-2014-029) extracted using new pp spectra •  Huge improvement in reducing systematic uncertainties

(earlier dominated by interpolation) •  RpPb is flat and consistent with unity

ATLAS-CONF-2016-108

See Petr Balek’s talk

Photon-jet correlations in pp and Pb+Pb •  Run 1 data established: isolated photon yields are not affected by

the medium à Use jet+photon events to measure jet energy loss in the QGP

•  Two observables: •  per-photon xJγ= jet pT / photon pT •  Δϕ – difference in azimuthal angle between a photon and jet •  In the pp system:

•  For xJγ: good agreement of data with PYTHIA8 simulation •  For Δϕ: good agreement for larger Δϕ, smaller Δϕ does not describe the data

due to lack of fragmentation photons in the MC

ATLAS-CONF-2016-110

See Brian Cole’s talk

Photon-jet correlation in Pb+Pb

•  In Pb+Pb 0-10% centrality: •  xJγ is shifted towards lower values and shape

is modified wrt predictions for all photon pT bins

•  Shape of the Δϕ distribution is consistent with that in pp collisions and in simulated Pb+Pb events

•  Observation qualitatively consistent with results at 2.76TeV

See Brian Cole’s talk

Charmonium production •  J/ψ and ψ(2S) production measured in pp and Pb+Pb at

5TeV •  Kinematic region: 9<pT<40 GeV and |y|<2 •  Tool to provide information on temperature and degree of

deconfinement of the QGP •  Test response of medium to prompt (cc-bar) and non-prompt (b

decay) components •  Use pseudo-proper decay time (τ) to distinguish between two

production mechanisms •  Non-prompt fraction consistent between three rapidity intervals in pp

and also between 5 and 13TeV data

ATLAS-CONF-2016-124

RAA for J/psi •  RAA measured for prompt and non-prompt

J/ψ production •  Strong J/ψ suppression •  As a function of pT: 0.2-0.4 for prompt (small

rise for high pT), 0.3 for non-prompt (flat) •  As a function of Npart: drop from 0.8-0.2 with a

similar trend for both components •  Striking similarity in the suppression pattern

See Sebastian Tapia’s talk See Jorge Lopez’ poster

ATLAS-CONF-2016-124

Suppression of ψ(2S) vs J/ψ

•  Ratio of RAA for ψ(2S) to J/ψ measured for prompt and non-prompt production •  Prompt: ratio ~0.5 i.e. ψ(2S) suppressed more than J/ψ due to lower binding

energy, less sensitivity to the recombination due to pT>9GeV requirement •  Non-prompt: ratio consistent with unity, consistent with production outside

the medium

See Sebastian Tapia’s talk See Jorge Lopez’ poster

ATLAS-CONF-2016-124

Light-by-light scattering in UPC events

•  Light-by-light (γγ à γγ) scattering: elastic scattering of two photons •  Tested indirectly in measurements of the

anomalous magnetic moment of the electron and muon

•  Despite its fundamental simplicity, no direct observation so far due to very small cross sections

•  Proposed as a possible channel to study •  Anomalous gauge couplings •  Contributions from charged SUSY partners

of SM particles •  Recent phenomenological studies/preditions

for SM rates: •  arxiv:1305.7142 (d’Enterria et al) •  arxiv:1601.07001 (Szczurek et al)

•  p,Pb is a source of EM fields

•  Very small Q2 of initial photons for PbPb à outgoing diphotons produced at small pT(γγ)

•  Box diagrams involve charged fermions (leptons or quarks) and W bosons

Light-by-light scattering in UPC events •  Search for signal diphoton candidates using:

•  Dedicated trigger: little activity in the calorimeter, no activity in the forward direction, little activity in the tracker

•  Two photons with ET>3GeV, Mγγ>6GeV, Aco=(1-Δϕγγ/π)<0.01 •  Exclusivity requirement: no tracks originating from the primary vertex

•  Excess in the data consistent with the light-by-light signal (arxiv:1601.07001) •  First direct observation of the light-by-light signal

See Piotr Janus’ poster

Measured cross section: σfid=70±20 (stat) ± 17 (syst) nb Predictions (arxiv:1601.07001): σfid=49±10 nb

Signal significance: 4.4σ, expected significance: 3.8σ

ATLAS-CONF-2016-111

Light-by-light scattering in UPC Overview of ATLAS results, Sept 23rd, 2016 16

Two back-to-back photons (ET=12 GeV and ET= 11 GeV) with Mγγ of 24 GeV with no additional activity

Soft physics

Flow harmonics in Pb+Pb at 5.02TeV •  Flow harmonics vn measured up to n=7

via SP/EP and 2PC methods •  Kinematic range: 0.5<pT<25GeV, |η|<2.5 and

centrality 0-80% •  First measurement of v7 in 0-60% centrality •  Significant v2 even at highest pT •  Very weak pseudorapidity dependence •  Results very consistent with Run-1

measurements at 2.76TeV

ATLAS-CONF-2016-105

Ridge in pp and p+Pb systems •  Ridge systematically studied in pp and

p+Pb systems •  5 and 13TeV pp data allow energy comparisons •  Events selected using the dedicated trigger

strategy - high-multiplicity track (HMT) triggers •  Ridge structure visible by eye in events with high

Nrec •  Fits to per-trigger yields include higher

harmonics (n≤4) for the first time •  p+Pb data reanalized w/o dependence on ZYAM

procedure

arxiv:1609.06213 Submitted to PRC

Ridge in pp and p+Pb systems

•  In pp v2 is flat with Nch and non-zero even for low Nch values

•  Agreement in all Fourier coefficients between 5 and 13TeV pp collisions

•  v2 and v3 harmonics larger in p+Pb collisions and raise monotonically

•  v4 tends to be consistent between the two collision systems

arxiv:1609.06213

See Sasha Milov’s talk

Ridge in pp and p+Pb systems

• Shape of pT-dependence of v2 is consistent between p+Pb and pp systems (scaled by 1.51)

•  v4/v22 ratio flat with Nch, higher for pp than p+Pb

•  Hint of the common origin of ridge in the two systems?

arxiv:1609.06213

Multi-particle cumulants in pp •  Multi-particle correlations in azimuthal angles measured in pp collisions

•  Two collisions energies explored: 5 (28 pb-1) and 13TeV (75 nb-1) •  Using well-established Q-cumulant method •  Expressed in terms of c2{4} which relates directly to elliptic flow v2{4}

•  Tool to assess collective nature of the pp system

Cumulants are extracted for reference particles (Ntrk

ref): •  0.3<pT<3 GeV •  0.5<pT<5 GeV using two methods: Method 1: fixed Ntrk

ref multiplicity (avoids multiplicity fluctuations) Method 2: events are selected using Ntrk(pT>0.4 GeV)

ATLAS-CONF-2016-106

Multi-particle cumulants in pp

•  Weak energy dependency to cumulants, however systematically higher c2{4} measured at 13TeV at low Ntrk, at higher Ntrk this trend is reversed

•  In Method 1: cumulants positive therefore no evidence of collective behavior •  In Method 2: negative cumulants over a broad range of Ntrk values •  PYTHIA confirms a negative contribution of fluctuations in Method 2 •  Conclusion: c2{4} in pp is sensitive to the choice of event class

•  Multi. fluctuations and non-flow could give a negative contribution to c2{4} •  Premature to conclude on collectivity w/o quantify influence of these effects

ATLAS-CONF-2016-106

Summary

•  Excellent performance of the ATLAS detector in Run 2 •  New results based on pp and Pb+Pb data at 5TeV have been

released •  RpPb for charged hadrons is consistent with unity up to pT~100GeV •  Photon-jet correlations confirm strong modification of xJγ with event

centrality •  Prompt and non-prompt J/ψ and ψ(2S) production shows a different

suppression pattern •  First direct observation of light-by-light scattering in UPC collisions,

signal significance at 4.4σ, good agreement with SM predictions •  Flow harmonics extracted to 7th order with SP and 2PC methods,

consistency with measurements from Run 1 •  Energy and system dependence in addition to Nch and pT dependence

of the ridge related observables may improve our understanding of small systems

•  Multiplicity fluctuations give a negative contribution to c2{4} à interpretation as collectivity may be premature

Contributions to HP2016 from ATLAS •  Talks:

•  Radim Slovak “Jet studies in heavy ion collisions with the ATLAS detector” •  Martin Rybar “Measurement of jet internal structure in p+p, p+Pb, and Pb+Pb collisions

with the ATLAS detector” •  Sebastian Tapia “Measurement of quarkonia production in 5TeV proton-proton and heavy-

ion collisions with the ATLAS detector” •  Qipeng Hu “Measurement of the suppression and v_{n} of heavy flavor muons in lead-lead

collisions with the ATLAS detector” •  Mirta Dumancic “Measurement of W and Z boson production in 5 TeV pp, p+Pb and Pb

+Pb collisions with the ATLAS detector” •  Brian Cole “Measurements of photon+jet production in 5 TeV pp and Pb+Pb collisions with

the ATLAS detector” •  Alexander Milov “Measurement of long-range azimuthal anisotropies and of forward-

backward multiplicity correlations and longitudinal flow with the ATLAS detector” •  Klaudia Burka “Measurement of azimuthal flow of soft and high-pT charged particles in

5.02TeV Pb+Pb collisions with the ATLAS detector” •  Petr Balek “Measurement of the nuclear modification factor for high-pT charged hadrons in

pPb collisions with the ATLAS detector” •  Posters:

•  Jorge Lopez “Measurement of J/psi production in 5.02TeV Pb+Pb collisions with the ATLAS experiment”

•  Piotr Janus “Light-by-light scattering in UPC Pb+Pb collisions at 5.02TeV” •  Radim Slovak “Triggering of Pb+Pb collisions at 5.02TeV with the ATLAS detector” •  Jakub Kremer “Performance of high-pT electron identification in lead-lead collisions at 5.02 TeV with the

ATLAS detector”

Back-up slides

Electroweak bosons in Pb+Pb system

•  Z, W and γ boson production has been studied in Pb+Pb system at 2.76TeV •  They are produced in hard scattering before the QGP is created

•  Their production yield per binary collision is flat with centrality and consistent with the NNLO or NLO calculations

•  They do not interact with the QGP and are established as standard candles for measurements tracking partonic energy loss

•  In case of W boson production, yields can be only described after including the isospin effect

•  Nuclear modifications to PDFs remain unclear within the precision reached in the Pb+Pb system

Overview of ATLAS results, Sept 23rd, 2016 27 Phys. Rev. Lett 110, 022301 (2013) Eur. Phys. J. C (2015) 75:23 Phys. Rev.C 93 , 034914 (2016)

See Mirta Dumancic’s talk

Jets in QGP •  A lot of progress in understanding jet production and partonic energy loss in the

medium based on Run-1 data •  Inclusive measurements à strong jet suppression up to RAA~0.5 in 0-10% •  Dijet asymmetry à strong modification of xJ with centrality •  Jet fragmentation à enhancement at low pT, suppression at intermediate pT and enhancement at

high pT •  etc

•  Measurements systematically extended to all collision systems •  Moving away from establishing various signatures to precision-like measurements

•  Latest development: unfolding of dijet asymmetry

See Radim Slovak’s talk See Martin Rybar’s talk

Phys. Rev. Lett. 114 (2015) 072302 ATLAS-CONF-2015-055 ATLAS-CONF-2015-052

Photon-jet correlations in Pb+Pb

• Background corrections: •  Combinatoric bkg contributes at low jet pT i.e. pT<50 GeV and is

strongly centrality dependent, almost negligible in 30-50% bin •  Dijet bkg contributes to a broad range of jet pT in particular

populating the region of jet pT > photon pT

Prompt and non-prompt J/ψ production in pp

•  Very good agreement of MC predictions with the data for prompt and non-prompt components in the 5TeV pp system

•  Non-prompt fraction consistent between three rapidity intervals and also between 5 and 13TeV data

ATLAS-CONF-2016-124

2PC flow harmonics in Pb+Pb Overview of ATLAS results, Sept 23rd, 2016 31

•  Flow harmonics vn measured up to n=6 via 2PC method •  Kinematic range: 0.5<pT<15GeV, |η|<2.5 and

centrality 0-80% •  Results presented for 2<|Δη|<5

•  Significant v2 even at highest pT •  Results very consistent with Run-1

measurements at 2.76TeV

ATLAS-CONF-2016-

See Klaudia Burka’s talk

SP flow systematics Overview of ATLAS results, Sept 23rd, 2016 32

2PC systematics Overview of ATLAS results, Sept 23rd, 2016 33

Fitting ridge components in pp Overview of ATLAS results, Sept 23rd, 2016 34

Fitting ridge components in p+Pb Overview of ATLAS results, Sept 23rd, 2016 35

Ridge in pp and p+Pb Overview of ATLAS results, Sept 23rd, 2016 36

Forward-backward correlations •  Measured with charged particles in |η|<2.4 and

pT>0.2GeV •  Three collision systems investigated: pp (13TeV),

p+Pb (5.02TeV) and Pb+Pb (2.76TeV) •  Novel approach to disentangle short- (SRC) and

long-range correlations (LRC) developed •  SRC is observed to be much stronger for opposite-charge

pairs than for the same-charge pairs, while LRC is found to be similar for the two charge combinations

•  After subtracting SRC, Csub is decomposed into a sum of products of Legendre polynomials

arxiv:1606.08170

Forward-backward correlations •  Both SRC and LRC contributions as a function of Nch follow a

power-law fit •  Highest contribution from SRC in pp than p+Pb and Pb+Pb •  LRC tend to be consistent between three collision systems

arxiv:1606.08170

High-pT exclusive dimuons •  Ultra Peripheral Collisions (UPC) with b>2R

•  Intense source of photons (~Z2) •  Well described by Weizsacker-Williams (EPA)

•  MC simulation: STARLIGHT 1.1 (integrated over nuclear excitation states)

•  Signal requirements: two good muons from the common vertex with unlike signs in the fiducial range: pT,1, pT,2 > 4 GeV, |η1|, |η2| < 2.4, Mµµ > 10 GeV

•  UPC dimuon pair should have pT < 200 MeV and thus small acoplanarity (Aco = 1 − |∆φ|/π) •  STARLIGHT does not include FSR photons à broadening

of the Aco distribution expected

ATLAS-CONF-2016-025

Multi-particle cumulants in pp •  Multi-particle correlations in azimuthal angles

measured in pp collisions •  Using well-established Q-cumulant method •  Expressed in terms of c2{4} which relates directly to

elliptic flow v2{4}

•  Two collisions energies explored: 5 (28 pb-1) and 13TeV (75 nb-1)

•  Tool to assess collective nature of the pp system

Cumulants are extracted for reference particles (Ntrk

ref): •  0.3<pT<3 GeV •  0.5<pT<5 GeV using two methods: Method 1: fixed Ntrk

ref multiplicity (avoids multiplicity fluctuations) Method 2: events are selected using Ntrk(pT>0.4 GeV)

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